Aqueous suspensions containing inorganic particulate material, for example an alkaline earth metal (e.g. calcium) carbonate or kaolin, are used widely in a number of applications. These include, for example, the production of pigment or filler containing compositions which may be used in paper manufacture or paper coating, and the production of filled compositions for paints, plastics and the like.
The inorganic particulate material typically has a known particle size distribution (psd), dictated by the intended end use in ways which are well known in the art. Generally, the required psd is obtained by a method which includes grinding the inorganic particulate material in an aqueous suspension. The suspension may contain a high (e.g. above about 50% by weight) or low (e.g. below about 50% by weight) content of the inorganic particulate solids.
Where the aqueous suspension has a high solids content, it is necessary to disperse the inorganic particulate material using an effective amount of a dispersing or deflocculating agent (dispersant). High solids grinding typically produces a relatively high proportion of ultra-fine particles (e.g. having an equivalent spherical diameter smaller than about 0.25 μm). To maintain an acceptably low viscosity of the suspension, one either needs to use relatively high levels of common dispersants, or lower levels of specialized—and thus relatively expensive—dispersants.
Where the aqueous suspension has a low solids content, a dispersant can be avoided. This enables the suspension to be dewatered cost-effectively, but problems can be encountered when mixing dispersant into the suspension later.
It is often desirable to produce a product which has a “steep” psd, in the sense that a large proportion of the particles are within a narrow size range. As used herein, a “steep” psd refers to a steepness factor greater than about 35, more particularly above about 40, steepness factor being defined as the ratio of the d30 equivalent spherical diameter (at which 30% by weight of the particles are finer) to the d70 equivalent spherical diameter (at which 70% by weight of the particles are finer), multiplied by 100.
In order to obtain this desired steepness, it is the present practice to grind the aqueous suspension at low solids in order to minimize the formation of ultra-fine particles. It is also desirable to perform the grinding stage in the absence of any dispersant in order to maintain a flocculated or aggregated state and so facilitate subsequent flocculated dewatering. In some cases, effective amounts of specialized dispersants are added to the dewatered, relatively high solids, suspension, to minimize flocculation and provide an acceptable viscosity for handling at that stage. It is also common practice to recirculate the water removed at the dewatering stage, to dilute the fresh feed. However, a number of problems can arise. For example, there can a significant loss in brightness (discoloration) due to corrosion products (e.g. iron-based corrosion products) from the system piping and equipment. Typically brightness values can be reduced by about 2-3 ISO units lower, with yellowness values about 1-2 ISO units higher. This discoloration is probably exacerbated in the low solids method, because the recirculated water becomes relatively highly aerated. Moreover, it can be difficult to introduce the dispersants in effective amounts at the high solids stages. In addition, the use of specialized dispersants can result, in materials which may then not be compatible in mixtures with materials including conventional dispersants such as polyacrylates.
The present invention aims to go at least some way towards overcoming the above problems, or at least to provide an acceptable alternative method of grinding an aqueous suspension of an inorganic particulate material.